T lymphocyte activation occurs through the clonally distributed T cell receptor or via non-clonally distributed cell surface molecules. The available set of alpha beta receptors is dictated by both positive and negative intrathymic selection events. The molecular basis of this selection, in particular, how bias for self-MHC presented peptides is achieved without corresponding deletions of these clones during establishment of self-tolerance, is unknown. This project uses cellular and molecular tools to study the development of the T cell repertoire and the activation of T cells by alpha beta and non-clonal surface receptors. The ability of cross-linked, non-clonotypic molecules to activate T cells has been studied using transfected cells. Previous data indicated that for Thy-1 and Ly-6-dependent stimulation, co-expression of TCR-CD3 complexes was required. In contrast, we found with both human cells and transfected mouse cells that anti-CD2 stimulation of IL-2 production can occur in the absence of surface CD3 expression, if the density of CD2 is sufficiently high. The relationship between CD2 level and stimulation is non-linear, and strong synergy is seen with co-expression of TCR-CD3. Mls is an uncharacterized set of molecules able to engage the receptors of T cells bearing certain Vbeta-containing TCR. This effect can be seen as intrathymic deletion in mice expressing the Mls product(s), or in the phenotype of proliferating cells from Mls negative mice stimulated with Mls bearing cells. The deletion events involve both CD4+ and CD8+ cells, whereas it was originally thought that stimulation of only CD4 + cells occurred among mature T cells. Recent data suggest CD8+ responses can occur as well. This latter finding raises questions about the ability of anti-CD4 antibody to "rescue" CD8+ cells in the thymus of Mls expressing mice. We have designed experiments to explore whether such cells are functionally tolerant but not deleted in this circumstance. Finally, based on a variety of data, we have proposed a specific molecular model for the intrathymic events of MHC-dependent peptide presentation to and recognition by alpha beta TCR that underlie the process of positive selection. Tests of this model, together with the above studies, should provide information about how the T cell repertoire develops and the role of various membrane molecules in T cell activation.